Journal of Petrology Advance Access originally published online on February 21, 2006
Journal of Petrology 2006 47(5):991-1015; doi:10.1093/petrology/egl001
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Very Low-grade Metamorphic Evolution of Pelitic Rocks under High-pressure/Low-temperature Conditions, NW New Caledonia (SW Pacific)


MINERALOGISCHPETROGRAPHISCHES INSTITUT DER UNIVERSITÄT BASEL, BERNOULLISTRASSE 30, CH-4056 BASEL, SWITZERLAND
RECEIVED JANUARY 21, 2005; ACCEPTED JANUARY 11, 2006
The PT gradient in a Late Eocene low-T high-P metamorphic belt in northern New Caledonia increases from SW to NE. Metapelites in the pumpellyiteprehnite and blueschist zones contain lawsonite, Mg-carpholite, Fe-stilpnomelane and Fe-glaucophane. Thermodynamic calculations indicate a progression of metamorphic conditions from less than 0·3 GPa and 250°C in a kaolinite-bearing rock in the SW, up to 1·5 GPa and 410°C in a lawsoniteglaucophane-bearing sample in the NE of the Diahot terrane. Through a multi-method investigation of phyllosilicates, organic matter and fluid inclusions, we demonstrate that the evolution of organic matter and illite crystallinity depends strongly on the evolution of the PT path with time. In addition, we show that the illitemuscovite b cell dimension provides a robust estimate of maximum pressure reached in low-temperature domains with polyphase metamorphic histories, despite subsequent high-temperaturelow-pressure events. Fluid inclusion study reveals an isothermal decompression in the Diahot terrane.
KEY WORDS: low-temperature/high-pressure metapelites; illite crystallinity; coal rank; illitemuscovite b cell dimension; New Caledonia
* Corresponding author. Present address: GeologischPalaeontologisches Institut, Senckenberganlage 3234, D-60054 Frankfurt am Main, Germany. E-mail: s.potel{at}em.uni-frankfurt.de
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